Field of the Invention
The present invention relates to a honeycomb filter.
Discussion of the Background
Exhaust gas discharged from an internal combustion engine such as a diesel engine contains particulates such as soot (hereinafter also referred to as PM). The PM adversely affects the environment and human bodies, which has been a recent issue. Since exhaust gas contains toxic gas components such as CO, HC, and NOx, there has also been a concern about the effect of the toxic gas components on the environment and human bodies.
To overcome this problem, various filters including a honeycomb structure (a honeycomb filter) formed of porous ceramics such as cordierite and silicon carbide have been proposed as exhaust gas purifying apparatuses. Such a honeycomb filter is connected to an internal combustion engine to capture PM in exhaust gas or to convert toxic gas components such as CO, HC, and NOx in the exhaust gas into nontoxic gas.
To enhance the fuel economy of an internal combustion engine and eliminate troubles due to an increase in the pressure loss during operation, various honeycomb filters have been proposed including those in which the initial pressure loss is low and those in which the rate of increase in the pressure loss is low when accumulation of PM reaches a certain amount.
Inventions of such filters are disclosed, for example, in WO 2004/024294 and U.S. Pat. No. 4,417,908.
FIG. 18A is a perspective view schematically showing a honeycomb filter disclosed in WO 2004/024294. FIG. 18B is a perspective view schematically showing a honeycomb fired body forming the honeycomb filter.
As shown in FIGS. 18A and 18B, WO 2004/024294 discloses a honeycomb filter 90 that includes a plurality of honeycomb fired bodies 100 combined with one another via adhesive layers 105 therebetween, and a peripheral coat layer 106 formed on the periphery of the combined honeycomb fired bodies, wherein the honeycomb fired bodies 100 each include exhaust gas introduction cells 102 each having an open end at an exhaust gas inlet side and a plugged end at an exhaust gas outlet side, and exhaust gas emission cells 101 each having an open end at the exhaust gas outlet side and a plugged end at the exhaust gas inlet side; the exhaust gas emission cells 101 each have a square cross section perpendicular to the longitudinal direction of the cells; the exhaust gas introduction cells 102 each have an octagonal cross section perpendicular to the longitudinal direction of the cells; and the exhaust gas emission cells 101 and the exhaust gas introduction cells 102 are arranged alternately (in a grid-like pattern).
Hereinafter, in the explanation of the embodiments of the present invention and prior art, a cell having an open end at an exhaust gas outlet side and a plugged end at an exhaust gas inlet side is simply referred to as an exhaust gas emission cell. In addition, a cell having an open end at an exhaust gas inlet side and a plugged end at an exhaust gas outlet side is simply referred to as an exhaust gas introduction cell, a first exhaust gas introduction cell, or a second exhaust gas introduction cell.
The simple term “cell” refers to both the exhaust gas emission cell and exhaust gas introduction cell.
Moreover, the cross section perpendicular to the longitudinal direction of the cells such as exhaust gas introduction cells, exhaust gas emission cells, or the like is simply referred to as the cross section of the exhaust gas introduction cells, the cross section of the exhaust gas emission cells, or the like.
FIG. 20A is a perspective view schematically showing a honeycomb filter disclosed in U.S. Pat. No. 4,417,908. FIG. 20B is an end face view schematically showing an end face of the honeycomb filter.
U.S. Pat. No. 4,417,908 discloses a honeycomb filter 110 in which all the cells have the same square cross section as shown in FIGS. 20A and 20B, and exhaust gas emission cells 111 each having an open end at an exhaust gas outlet side and a plugged end at an exhaust gas inlet side are adjacently surrounded fully by exhaust gas introduction cells 112 and 114 each having an open end at the exhaust gas inlet side and a plugged end at the exhaust gas outlet side across cell walls 113. In regard to the cross-sectional shape, a side of the exhaust gas introduction cell 112 faces the exhaust gas emission cell 111 across the cell wall 113, while the exhaust gas introduction cell 114 is arranged such that the corners thereof face the corners of the exhaust gas emission cells 111, and thus none of the sides forming the cross-sectional shape of each exhaust gas introduction cell 114 faces the exhaust gas emission cell 111.